Spin blockade occurs when an electron is unable to access an energetically favourable path through a quantum dot owing to spin conservation, resulting in a blockade of the current through the dot 1-6 . Spin blockade is the basis of a number of recent advances in spintronics, including the measurement and the manipulation of individual electron spins 7,8 . We report measurements of the spin blockade regime in a silicon double quantum dot, revealing a complementary phenomenon: lifetimeenhanced transport. We argue that our observations arise because the decay times for electron spins in silicon are long, enabling the electron to maintain its spin throughout its transit across the quantum dot and access fast paths that exist in some spin channels but not in others. Such long spin lifetimes are important for applications such as quantum computation and, more generally, spintronics.Semiconductor quantum dots or 'artificial atoms' provide highly tunable structures for trapping and manipulating individual electrons [9][10][11] . Such quantum dots are promising candidates as qubits for quantum computation [12][13][14] , owing in part to the long lifetimes and slow dephasing of electron spins in semiconductors 7,15 . Si quantum dots are predicted to have especially long lifetimes and slow dephasing, due to low spin-orbit interaction and low nuclear spin density 16,17 . In the past several years, much activity has focused on the development of quantum dots in Si/SiGe (refs 18-22) and recent advances in materials quality and fabrication techniques have enabled the observation of coherent spin phenomena in such quantum dots 23 . Spin-to-charge conversion, in which spin states are detected through their effect on charge motion, enables measurement of individual electron spins in quantum dots 15 . Spin blockade is the canonical example of spin-to-charge conversion in transport, where charge current is blocked in a double quantum dot by a metastable spin state. The blockade occurs when one electron is confined in the left dot and a further electron enters the right dot forming a spin triplet state T(1,1) (Fig. 1a). Exiting the dot requires reaching the triplet T(2,0), with both electrons in the left dot, a state that is higher in energy. The electron is thus trapped in the right dot, unless relaxation from T(1,1) to S(1,1) occurs, opening a downhill channel through S(2,0). As we show below, this aspect of spin blockade in Si is virtually identical to that previously observed in other systems 1-3 . The unexpected effect presented here is lifetime-enhanced transport (LET). The energy level diagram for LET is the same as for spin blockade, except that current flows in the opposite direction (Fig. 1b). Flow through the triplet channel is now energetically downhill, whereas flow through the singlet channel is very slow, because it requires either an uphill transition or tunnelling directly from the left dot to the right lead. Transport current will be observable only if electrons flow almost exclusively through the triplet channel,...

The site preference and elastic properties of Fe-, Co-, and Cu-doped Ni 2 MnGa alloys are investigated by using the first-principles exact muffin-tin orbital method in combination with coherent-potential approximation. It is shown that Fe atom prefers to occupy the Mn and Ni sublattices even in Ga-deficient alloys; Co has strong tendency to occupy the Ni sublattice in all types of alloys; Cu atoms always occupy the sublattice of the host elements in deficiency. For most of the alloys with stable site occupations, both the electron density n and the shear modulus C can be considered as predictors of the composition dependence of the martensitic transition temperature T M of the alloys. The physics underlying the composition-dependent C are discussed based on the calculated density of states.

We analyze electron transport data through a Si/SiGe double quantum dot in terms of spin blockade and lifetime-enhanced transport (LET), which is transport through excited states that is enabled by long spin relaxation times. We present a series of low-bias voltage measurements showing the sudden appearance of a strong tail of current that we argue is an unambiguous signature of LET appearing when the bias voltage becomes greater than the singlet-triplet splitting for the (2,0) electron state. We present eight independent data sets, four in the forward bias (spin-blockade) regime and four in the reverse bias (lifetime-enhanced transport) regime, and show that all eight data sets can be fit to one consistent set of parameters. We also perform a detailed analysis of the reverse bias (LET) regime, using transport rate equations that include both singlet and triplet transport channels. The model also includes the energy dependent tunneling of electrons across the quantum barriers, and resonant and inelastic tunneling effects. In this way, we obtain excellent fits to the experimental data, and we obtain quantitative estimates for the tunneling rates and transport currents throughout the reverse bias regime. We provide a physical understanding of the different blockade regimes and present detailed predictions for the conditions under which LET may be observed.

Density functional theory (DFT) calculations are used to investigate the correlation between the uniaxial negative thermal expansion (NTE) and negative linear compressibility (NLC) behaviors in Ag3[Co­(CN)6]. First, we reproduce the uniaxial-NTE and NLC behaviors under temperature- and pressure-field. And then the temperature dependence of elasticity is studied. The abnormal nature of elastic constants C 33 and C 11+C 12 as the function of temperature is predicted. The hardening of phonon modes (below 568.7 cm–1) with increasing temperature can be as an indicative for abnormal physical properties of Ag3[Co­(CN)6]. Through analyzing the vibration mode with the strongest phonon hardening, the deformation of wine-rack motif in anisotropic framework can be identified as the mechanism that leads to the coexistence of uniaxial-NTE and NLC in Ag3[Co­(CN)6]. The response of phonon group velocity along c-axis is intense and quicker than that of a-axis on heating and on compression, facilitating c-axis itself to be the carrier of abnormal uniaxial-NTE and NLC properties.

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